I did the trip a year ago, so I'm not up to date on the security situation. Fuel is no problem, in Kenya you fuel up at Isiolo, then Marsabit and at Moyale (or preferably after the border on the Ethiopian side). Those places have guest houses as well. Likely you will do the 250k from Isiolo to Marsabit on one day and the 250 from Marsabit to Moyale the following day. I think the border closes at 6pm, so try to get an early start. Ethiopia is very convenient for travel, fuel and cheap guest houses will be readily available in every town. Make sure you carry enough water, there is nothing between towns in northern Kenya. Good luck and enjoy.